Swap space on a Linux box is an area on disk that is used to hold inactive memory pages. This occurs when the system needs more memory then is currently available, so it swaps these inactive memory pages to disk.
To create additional swap space on the fly you are either going to need a spare disk or free partition on a disk that you can use.
First, using fdisk, you will need to make sure that the partition type for the disk (or partition) is set to 82.
Then setup the swap area using mkswap. In this example I am using /dev/sdb2, but your setup is bound to be different.
Then determine the UUID of the new swap space.
Then add an entry to the /etc/fstab, so that the swap space is mounted at boottime. The show in the example below is the output of the blkid command above.
>UUID=7b05f0a9-18d5-42e5-b259-78ba3a8cc1b7 swap swap defaults 0 0
Then activate your new swap space
Then check to make sure everything worked by checkign for your new swap partition in the output of the command below.
/dev/sdb2 partition 4193276 7480 0